Embodiments of the present invention relate to generating and writing a circuit design pattern on a substrate with a shaped particle beam.
Pattern generators are used to write patterns on substrates, such as masks for semiconductor fabrication, semiconductor wafers, displays and circuit boards, magnetic bubble fabrication, and optical data storage media fabrication. For example, patterned lithographic masks are used to fabricate integrated circuits, printed circuits (PCB), displays, and other patterned boards. The lithographic mask is made up of a plurality of pattern elements formed on a radiation permeable plate. The patterned elements are used to transfer a circuit design pattern into a substrate, such as a semiconductor wafer or dielectric. A typical process for fabricating a lithographic mask includes, for example, (i) forming a layer of a metal-containing material on a radiation permeable plate, (ii) forming a resist layer on the metal-containing layer to create a blank lithographic mask, the resist layer being sensitive to a particle beam such as an electron or ion beam, (iii) writing a pattern on the mask by selectively exposing the blank lithographic mask to the modulated particle beam, (iv) developing the exposed material to reveal a pattern of features, (v) etching the revealed portions of the metal-containing material between the resist features to transfer the pattern captured in the resist features into the metal-containing material, and (vi) stripping residual resist from the lithographic mask.
The pattern is written by selectively exposing an energy sensitive resist layer on the substrate to a particle beam. During writing, the particle beam is broken into discrete units, commonly referred to as flashes, which form a portion of the pattern that is exposed on the substrate during an exposure cycle. While the particle beam is modulated to form flashes and deflected across small dimensions, the substrate is moved in a plane by a substrate support, causing the modulated particle beam to write the pattern on the substrate. A particle beam such as an electron beam can be formed in a electron beam column having discrete components that focus, blank and deflect electrons generated from a source to write an electron pattern on a substrate, exemplary columns being in, for example, U.S. Pat. No. 6,262,429 to Rishton et al.; U.S. Pat. No. 5,876,902 to Veneklasen et al.; U.S. Pat. No. 3,900,737 to Collier et al. and U.S. Pat. No. 4,243,866 to Pfeiffer et al.; all of which are incorporated herein by reference in their entireties.
The modulated particle beam is moved and flashed across the substrate using a scanning system, such as for example, raster, vector, or hybrid raster-vector scanning. The beam and substrate are moved relative to one another so that the beam traverses across the substrate in linear strip-wise motion (e.g. raster beam scanning), in vector based steps (vector scanning), or in a combination of vector and raster scanning. For example, in one raster scan method, called raster Gaussian beam scanning (RGB), a particle beam is scanned in successive parallel lines to cover all the pixel cells of a pixel grid across the substrate. The circuit design pattern is retrieved and sampled to create a gray pixel map from which geometric shapes in the form of particle beam flashes are constructed. The substrate is exposed to the flashes one pixel site at a time along the scan trajectory by moving the substrate along a plane perpendicular to the direction of the particle beam while the beam is flashed by blanking. In this process, the flash area is roughly the size of the address unit of the pixel grid. Because the flashes have a roughly Gaussian profile adjacent flashes blend smoothly together. RGB has the advantages of good pattern resolution and predictable writing speed. However, the Gaussian flash profile limits feature resolution and critical dimension (CD) uniformity. RGB resolution can be improved by reducing the basic address unit but this quadratically lengthens writing time. Overlapping flashes and gray modulation can be used to achieve finer edge placement, but fundamental limitations of feature resolution and CD uniformity are still a problem for fine features.
In a vector scan method, called vector shaped beam (VSB) scanning, a particle beam with a variable shaped beam is moved across the substrate directly to locations specified by vector coordinates and then flashed once over those locations. The circuit design pattern is divided into a series of rectangles and parallelograms, and the particle beam is then controlled to move directly to only those pixel cells that require exposure and then flashed to expose the pixel site with the desired shaped beam. The beam is controlled to have different flash shapes and sizes which can expose multiple pixel cells at the same time. Thus, the original pattern is translated to a vectorized representation having a list of pixel locations and corresponding beam flash shapes and areas. The flash area is independent of the addressing of the vector and the flash profile for a given flash area is much steeper, that is it has sharp edges at which the energy sharply dissipates in intensity, than the edges of a Gaussian beam which gradually dissipate in a Gaussian curve. This provides better resolution. Also, in VSB, the beam traverses directly to the area to be written while unexposed areas are skipped allowing the total exposure time to be reduced by the percentage of unwritten area. While VSB improves feature resolution and critical dimension uniformity, it has relatively slow flash rates because of the high precision digital to analog converters (DAC) required for vectoring the beam.
In one hybrid raster-vector scan method, commonly known as raster shaped beam (RSB) scanning, a shaped particle beam that can provide flashes having different shapes and sizes, is line scanned across the substrate. As with VSB, the flash profile is steep, that is it has sharp edges, yielding good resolution and CD uniformity; and as with RGB, the rasterized scan yields good placement accuracy. Furthermore, the flash rate can be much higher than that employed in VSB, because only low resolution DACs are required. However, RSB unfortunately retains the disadvantage of RGB in that further improvements to feature resolution quadratically lengthen printing time because each flash must be contained within a pixel grid cell.
In order to write newer circuit design patterns having features with increasingly smaller dimensions, it is desirable to have a writing method and strategy which provides high pattern resolution coupled with faster writing speeds. Higher resolution is needed because newer circuit designs have features with dimensions on the order of less than 50 nm which is more 2½ smaller than conventional feature sizes of over 130 nm. Vectorized scanning methods provide faster writing speeds by maximizing the flash area which is the area covered by a single flash. However, larger flash areas, of for example electron beams, can lead to larger electron blur because of the electron to electron interactions within each flash that arise from the higher beam current carried by the larger flash. This results in poor definition of the edge of the flashed area resulting in lower pattern resolution.
Thus it is desirable to have a pattern generator and writing method that provides higher resolution for circuit designs with finer dimensions. It is further desirable to have a writing system that can write a particle beam with improved resolution without sacrificing writing speed. It is also desirable to have a writing strategy that may be implemented using many different scanning systems.